1. Field of the Invention
This invention relates generally to the treatment of wastewater sludge, and more particularly, to a method of treating wastewater residuals and similar organic waste material to produce a material which may be beneficially reused.
2. Description of the Prior Art
The need to treat wastewater sludge is well known in the art together with various methods designed to permit sludge beneficial reuse. Alkaline treatment is one such method and is an effective means for stabilizing sludge by increasing the pH of the sludge to 12 or higher. Typically the use of lime requires stabilization dosages of approximately 0.2 pounds of lime per pound of solids. If the sludge is also subjected to temperatures of 70.degree. C. or over for at least 30 minutes, the result is a complete disinfection commonly referred to as pasteurization resulting in the destruction of pathogens and indicator organisms making the sludge suitable for general plant nutrient supplementation and soil conditioning without a site specific application permit. The Environmental Protection Agency recognizes this type of process as a Process That Further Reduces Pathogens (PFRP) and allows unrestricted contact with biosolids so treated.
When calcium oxide (quicklime or CaO) or magnesium oxide (MgO) is added into sludge, the calcium oxide reacts exothermically with the free water of sludge to create calcium hydroxide (hydrated lime) plus heat. Calcium hydroxide may also continue to react with carbon dioxide in the atmosphere to form calcium carbonate. Although this carbonation reaction is exothermic, the typical heat available is modest due to the slow rate of the reaction.
Either method requires a high dosage of lime to generate a disinfecting heat by the exothermic reaction known as the heat of hydration. In order to achieve pasteurization temperatures of 70.degree. C., typical dosages are one pound of lime per pound of dry solids at a 20% initial solids concentration. The heat of hydration for lime is equivalent to approximately 491 BTU's per pound of lime. Additional chemical heats are available, such as carbon dioxide or the heat of carbonation, which release heat equivalent to approximately 780 BTU's per pound and may be used to reduce the lime requirement.
Numerous patents address the need for stabilization and sterilization. U.S. Pat. No. 4,270,279 issued to Roediger also discloses another method of alkaline stabilization of dewatered sludge by the use of quicklime dust which results in the formation of pellets. The quick lime dust reacts exothermically with the surface of the pellets resulting in a product that can be used as agricultural fertilizer.
U.S. Pat. Nos. 4,306,978 and 4,997,572 issued to Wurtz discloses a method of producing sludge pellets using sludge stabilized by lime by the inclusion of calcium oxide or similar alkaline materials and combinations thereof, with dewatered wastewater sludge cake in a compartmented reactor. The mixture reacting with carbon dioxide providing disinfection, stabilization, and pelletizing of the sludge. Wurtz requires that the dewatered sludge is provided in a sludge cake which is blended with calcium oxide to form sludge particles. The sludge particles are then mixed with calcium oxide to form calcium hydroxide raising the pH of mixture to a range of 11 to 13. The sludge is then placed into a second compartment wherein the reaction of calcium oxide with free water continues to form calcium hydroxide and raises the pH of the mixture to a selected pH. The sludge is finally placed into a third vessel to react with induced calcium hydroxide and carbon dioxide gas forming calcium carbonate to provide the basis for pellet formation.
One problem with the prior art, of which this invention addresses, occurs during the addition of lime as a primary step for purposes of stabilization and/or disinfection. The receipt of wastewater is not a fixed or known variable causing the addition of lime to become an unknown variable. The result is an uninhibited and often costly addition of lime added to the sludge until the desired pH stabilizing occurs. Another problem is the lack of odor control which is not addressed by the prior art. The control of odor directly affects the acceptablilty of a wastewater treatment process.
Thus, what is needed in the treatment of wastewater and unbeknownst to the prior art, is a method of biosolids conditioning at a low pH before the addition of lime and the various benefits that are derived therefrom.